1. Field of the Invention
The present invention generally relates to solar cells, and more specifically to a solar cell assembly that includes a bypass diode that is positioned in a recess on the backside of a solar cell for anti-parallel connection with the solar cell.
2. Description of the Related Art
Arrays which may include hundreds or thousands of solar cells bonded to a solar panel are used to provide electrical power for a variety of applications such as spacecraft. In most applications in which solar cells are used, size and weight considerations are very important. A solar cell includes a flat photovoltaic wafer made from n-type or p-type crystalline semiconductor material, such as silicon, gallium arsenide or germanium, in or on which a thin surface layer of the opposite conductivity type is formed. The interface between the surface layer and the main or bulk region of the wafer defines a semiconductor junction. A current collector grid metallization is formed on the surface layer and a metal layer is formed on the back surface of the wafer. Illumination of the surface layer causes a liberation of charge carriers, including electrons and holes in the region of the semiconductor junction, which migrate toward opposite surfaces to establish a potential across the solar cell.
Solar cells are typically modeled as diodes that respond to illumination by becoming forward biased and establishing a voltage across the cell. A silicon solar cell produces about 0.6V, while a gallium arsenide cell produces about 1.0V. A relatively new class of multijunction solar cells, formed from a combination of group III and group V materials and commonly referred to as Advanced "III-V" cells, produce a somewhat higher forward voltage. When the solar cell is in a circuit and is not illuminated, the current flow through the cell becomes limited, which causes the cell to become reverse biased. If the reverse bias voltage is high enough the solar cell may break down and become permanently damaged. Silicon cells have a typical breakdown voltage of approximately 60V with a minimum of approximately 35V. Gallium arsenide cells have a much lower breakdown voltage, nominally 5V and as low as 1V, and hence are more susceptible to damage. The Advanced "III-V" cells can break down at even lower voltages.
The solar cells may be connected in series strings to provide a desired voltage, in parallel to provide a desired current, or in a series-parallel combination. When all of the solar cells are illuminated, they each produce their respective voltage or current outputs which sum together to maintain the desired overall output. However, if one or more of the solar cells becomes shadowed those cells become reverse biased. For example, in a spacecraft an antenna may cast a shadow across the array.
The effect of shadowing a solar cell in a series string depends upon the specific characteristics of the cell. If the cell has a very low reverse current, reverse biasing the cell will effectively force the string output to zero. Conversely, if the cell breaks down at a relatively low reverse voltage, the effect of shadowing a cell on the string output is reduced. However, the cell can be permanently damaged.
Bypass diodes, typically of silicon, are used to minimize output losses and to protect cells when they become shadowed. Bypass diodes can be connected across single cells, across strings of cells, or across rows of parallel-connected cells. As shown in FIG. 1, the bypass diode 10 and solar cells 12 are connected in an anti-parallel configuration such that the bypass diode is reverse biased when the solar cells are illuminated. Bypass diodes that have very low reverse currents are preferred to avoid reducing current in the solar cell during normal operation, which would reduce power efficiency. When the cell becomes shadowed, the current flow through the cell is limited, causing the cell to become reverse biased. This causes the bypass diode to become forward biased and conduct so that the current in the string can continue to flow. The bypass diode also limits the reverse bias voltage across the cell so that it does not break down and become permanently damaged. The voltage output V.sub.out of the string of cells is reduced by the voltage that would be produced by the non-illuminated cells and the voltage drop across the bypass diode.
The "Solar Cell Array Design Handbook," by H. S. Rauschenbach, Van Nostrand Reinhold Co., pp. 300-302 (1980) discloses three types of bypass diodes: conventional rectifier diodes, rectifier wafers and integral diodes. The conventional rectifier diodes are wire bonded to the solar panel adjacent the solar cells. In strings of silicon cells, one bypass diode per every seventy cells is sufficient to limit the maximum reverse voltage to be less than 35V so that the cells do not break down. The additional space on the solar panel, weight, time, and cost of installing one diode per seventy cells is not significant. However, in strings of gallium arsenide cells, one bypass diode per cell would be required to ensure protection. Instead, typically one diode per five cells is used and the cells are screened to remove those cells that break down at less than 5V. This typically results in a loss of between 10% and 30% of the cells. Furthermore, wire bonding the diodes to the surface of the solar panel generally increases the overall size, and thus weight, of the panel by approximately 15%.
In Rauschenbach, rectifier wafers are used to protect solar cells in densely packed arrays in order to conserve space. The rectifier wafers are placed underneath the solar cells but are not directly attached to the cells. Solar cells are bonded to the solar panel along an adhesive bond line that is typically 0.004 inches thick. The placement of the wafers between the solar cells and the panel necessitates the use of a thicker bond line. This increases the cost of manufacture, substantially increases the weight of the solar panel, and may weaken the mechanical connection between the solar cells and panel.
Rauschenbach discloses that a significant improvement over the flat wafers can potentially be achieved by using solar cells with integral diodes. An integral diode is formed by fabricating the diode into the solar cell itself. This can be accomplished for silicon solar cells but is very difficult for gallium arsenide cells. Furthermore, integral diodes are characterized by a significant reverse current that tends to degrade solar cell performance.